On the ShowEdit
Since their third child was mentioned in "Forever Charmed" it is assumed that this is the same child, a third daughter, which Piper claims she also saw in a vision. The first initial is also assumed since Phoebe told Prue that she had decided to continue the tradition of using the letter "P" to start names and both P.J. and Parker have names starting with "P".
In the ComicsEdit
She hasn't been born yet or seen yet, so she hasn't said anything.